Means for measuring mechanical forces and their time derivatives



Julyzl, 1959 o. DAHLE ETAL A 2,895,332

MEANS FOR MEASURING MECHANICAL FORCES AND THEIR TIME DERIVATIVES Filed Dec. 14, 1954 2 Sheets-Sheet 1 I0 I I v 9 /n venfors. QRVAR DAHLE and 5 IRS T DAHL E, Nee ANDERSSON Mtornei.

July 21,1959

Filed Dec. 14. 1954 HLE ETAL 2,895,332

0. DA MEANS FOR MEASURING MECHANICAL FORCES AND THEIR TIME DERIVATIVES 2 Sheets-Sheet, 2

02ml? DAHLE' and 51/?6/ 7' DAHL E, NeeA/vDFRSSoN 5y k figfomey United States Patent Sweden, assignorsto'iAllnianna svenska.Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Application December 14,4954; seinrivoa 475,122 Claims priority,: application Sweden December- 28} 195a 9 claims. f ctive- 141 In methodsof measuring mechanical forcesas. hitherto normally used, the point. of applicatiomof the forceto be measured i'sCdisplaced. a certain distance,.-.the measurement being based upon this'displac em'ent. .The-measure1 ment of mechanical forces in 'mechanically alfigid con structions where the active forces;do not--.bring about any appreciable deformation, has therefore hitherto=involved great difliculties. An-example=hereof-is the measurement of the thrustbetween. the rollers in a mill frame. It is evident 'also, that-the weighing. of heavy weights, for instance ore trucksawouldaofier greatad-. vantages, if the weighinggcould be performed by means of astructure without moving-parts, q p p i In the first mentioned case, due to thel extremely small deformation involved, it hasheretoforebeen necessary to use extremely sensitivemeasuring.devices, for'measuring the displacementof the pointof application of the force. Such a device is-the so calledstrain gauge, which is based, upon the variation in the resistance of a. thin wire as it is subjected to mechanical tensile or; compressive stresses. The measuremengq-therefore, is performed electrically, as is alsothe' casei in 'somenothenmeasuring methods heretofore suggested. All thesemethods, however, have the drawback that the available ,powers resulting from measurement are very small, so that. amplification often has -to beemployedi-beforet therrneasuring result can be read or recorded by an instrument or fed to a control device. v V g The present invention. provides means for. the solution of the above mentioned measuringmroblem and for obtaining measuring results havingvsufliciently wgreat powers for measuring orrecording the sarne by means of electrical instruments or for causing such powers to influence a control meanswithout anvgamflification. The invention is based on a propertyiofi magnetic ma-. terial which is usually called magneto-.striction, i.e. when magnetic material is subjected to mechanical,strain,...the permeability is altered inthe direction .of. .the .st rain. Two different kinds of magneto-striction.may-occur, viz. the positive and the negative magneto-striction. .Ylllhe positive magneto-striction which occurs for instance. in Permalloy, means'that the permeability "increases for tensile stressing, whereas the negative magnetor stiiction occurring for instance, in-nickel means that the:.permeability decreases 'for tensile stressing Ordinary. iron. has a positive or negative. magneto-striction,depending .on thedegree of magnetization. I I 'X H The invention. provides a structure substantially. characterized by -a measuring body whichisentirely. or partly surrounded by two coils, thewinding-planes of which are arrangedso thatthe rnutual inductanceo f coils is zero, when the measuring bodyis unstressed. The one coil, connected to ran-electriccurrent source, mag.- netizes the material thus producing a voltage thebther coil .due to the differingpermeabilitvin. diiferent directionsof the material arising when the body isisubjected to forces to be measured. The voltage across the latter coil will then be a measure of the mechanical strain a v 2,895,332 Patented July 2 1959 I the invention. in unstressed and stressed conditions; re-

spectively,

. .Figs. 3 through 6 show different forms of--'measuring devices according to the invention. Thei-modc of action of the invention isillustrated by Fig. 1;. -which-shows a measuring body 1' of magnetic material having four openings or ducts 2, 3, 4.and 5. In ;two off;the ducts (3 and 4) there is 'wound'an. exciting. coil 6 connected toa source 12 of: electric cur- Ient'sourceand in the other two ducts (2 andS) is a measuring coil 7 connected to a voltage sensing. device 13. The coils are shown in the figure with only-one turn each, for-the sake of simplicity, but obviously there maybe-more turns. When the exciting coil 6 is traversed by electric current in the direction shown on the figure,

a1magnetic flux 8 is produced in the measuring body.-

1 crease. inlthe horizontal.direction.:This results in a change inr the .directionof the magneticfiux lines in. the measuringlbody,.land, as theeflect of thechanges in the permeability isfmost pronounced atthe center of the, measuring body where the flux density is greatest," the fiuxilines in the stressed measuringbody will be deformed as shown in Fig. 2 in which the. measuring coil 7 embraces a part of the flux. so that a voltage is induced init, the magnitude'of the voltage depends on the. angle through which the flux lines have been turned.-

. Sincefthe voltage induced in the measuring coil 7 is proportional to the time derivative of the magnetic flux, it' is necessary that for measuring the magnitude of the mechanical force P the exciting current in the coil 6 is Jan alternating current. If the exciting current were a, direct current, the voltage induced in the measuring coil 7 would be a measure for the time derivativeofthe mechanical force P.

The practical construction of the measuring body 'need not, of'clourse, follow exactly the embodiments shown 3 schematically in Figs. 1 and 2, but it maybe shaped in different manners, such as an octagon as shown in Fig. 3,

for'instance Also the coils need notbe placed within openings or ducts in the inner part of the measuringibody but they may embrace it, as shown in Fig. 3.. In the last mentioned case, the measuring body becomes mechanicauy: stron and easy to manufacture, but the magnetic flux. has to traverse in this case rather large air. gaps, so that the exciting coil draws a heavy current. and rather small sensitivity is attained. Furthermore it is .not ne cessary that the Winding planes of the coils lie at right angle to eachfother, but it is-essential that the mutual inductance of the coils is zero whenthe measuring body is unstressed.

If the excitation is effected by alternating current, it is of course suitable that the measuring body shall be. compos ayef a; plurality of thin sheets orlaminae of magnetic material stacked one upon another. The measuring body maythen be held together by bolts inserted within: punchedhbles in 'thc laminae. These holes and bolts, however, would disturb the magnetic flux. distribution in thenieasuring body, it is thus better to place one "rigid 2,s95,ss2 V r q r plate on each side of the measuring body, between which plates the sheets of the measuring body are pressed together by bolts located outside the measuring body itself. Fig. 4 shows such a construction, wherein 1 is the measuring body, held between two plates 9 by means of a plurality of bolts 10. This construction has however the drawback that a certain mechanical hysteresis effect occurs due to the friction forces occurring between the plates and the measuring body. This hysteresis, however,

can be eliminated by bonding the sheets with a suitable synthetic resin. 4

Obviously, the applicability of the invention is not restricted to compressive forces, as shown in Fig. 2, but tensile forces have, as will be seen from the secondparagraph of the description, a similar influenceon the magnetic material. The measuring body should of course be so shaped that the tensile force can be applied. It is also possible to apply the mechanical force parallel to the plane of the coils, i.e. at right angle to the plane of the sheets so that bending stresses occur in the measuring body.

In the measurement of compressive forces, it is very difficult to obtain an even distribution of the applied force over the whole cross-section of the measuring body as shown in Fig. 2. The results of measurement are greatly dependent on whether the force really is evenly distributed over the surface, and the surfaces of the measuring body and the surfaces between which it is pressed must be very smooth, parallel, and absolutely rigid; The sensitivity of the distribution of the load involves the risk that the results of the measurement cannot be. reproduced. It will also be seen from Fig. 2 thatthe compressive stress obtained in those parts of the measuring body which lie outside the coils has anaction opposed to the desired action. Another drawback is that the material around the ducts 2, 3, 4 and in which the coils are inserted is subjected to the strongest mechanical stresses. Should therefore the measuring body be overloaded, the material around these ducts would flow with the result that permanent stresses would be produced which would alter the magnetic properties of the measuring body and also the results of measurement. These disadvantages may be eliminated by providing both surfaces of the measuring body, which are subjected to the mechanical force, with projections on which the mechanical force acts. Fig. 5 of the drawing shows in principle such a measuring body, in which 1 designates the body itself having two projections 11. These serve as equalising distance pieces so that the force is evenly distributed at their base independently of how the force is applied to their free ends. At the same time the force by this construction is concentrated to the center of the measuring body, whereby greater sensitivity is obtained. The projections are suitably dimensioned so that on the occurrence of an overload the flow takes place in them earlier than at the edges of the holes, so that they serve as a mechanical overload protection. Therefore itis also suitable that the transition between the measuring body and the projections is made gradual, since otherwise indefinite mechanical stresses would be obtained within the magnetically active parts of the measuring body. This construction is also suitable for the measurement of tensile forces, if the projections are shaped so that a tensile force can be applied.

It is of course also possible to increase the dimensions of the measuring body in proportion to the'magnitude of the force to be measured. As the measuring body, however, must have a substantially symmetrical cross-section at right angles to the slots, this involves a too high construction for large forces. Therefore it is more suitable to place a plurality of small measuring bodies side by side, so that each of them takes up a part of the mechanical force, and to series connect their measuring coils for obtaining a measure for the sum of the forces taken up by them. In order to get a mechanically stable and strong construction, a plurality of measuring bodies lying side by side may be punched in the same sheets, so that when they are assembled, a plurality of measuring bodies forming a common mechanical unit is obtained. The exciting coils should then be suitably connected so that the different measuring bodies-do not disturb each other magnetically. Fig. 6 shows schematically such a construction, wherein .both the exciting and the measuring coils are series connected in such a manner that the magnetic:fields do not disturb each other and that the results of measurement from the different measuring bodies are added.

If the coils of a plurality of measuring bodies are series connected as described above, it is necessary for obtaining proper adding that either all the measuring bodies are influenced by equal mechanical forces, which is diflicult to obtain, or that a constant ratio exists between the voltage induced in the measuring coil and the mechanical force applied. This rectilinear relationship between force and measuring voltage may be obtained with rather good accuracy, ifa suitable value of the exciting current is chosen, and particularly if only the fundamental wave of the voltage induced in the measuring coils is measured. Suitably the exciting current is chosen so heavy that sat uration occurs in those parts of the measuring body which are embraced by the coils.

Due to the above mentioned projections on the measuring'body, a magnetic asymmetry is produced in consequence of which a certain voltage is induced in the measuring coil, even if the measuring body is mechanically unstressed. This zero-voltage may also depend on other factors, for instance on the asymmetry of the holes in the body, or on the fact that the sheets are-manufactured from rolled material, which always possesses a certain magnetic anisotropy. It is, however, possible to eliminate this zero-voltage by changing the angle between the "winding planes of the coils and thereby their mutual inductance. 7

However, it is also rather easy to compensate electrically the zero-voltage by an additional voltage, which of course has to be variable with respect to amplitude and phase angle. This additional voltage may be obtained from an external voltage source but also from an additional winding inserted in the same slots as the exciting coil.

We claim as our invention:

1. A means of the character described for measuring the time derivative of a mechanical force, comprising a measuring body of magnetostrictive material, means for applying said force to said measuring body, two coils embracing atleast a part of said body, the winding planes of said coils being disposed at right angles to eachother and at substantially 45 to' the direction of said force and so arranged that the mutual inductance of said coils is substantially zero when the measuring body is mechanically unloaded, a direct current source connected to one of said coils which serves as an exciting coil, and a voltage sensing means connected to the other coil which serves as a measuring coil.

2. A means of the character described for measuring mechanical forces comprising a measuring body composed of a plurality of sheets of magnetic material having holes therein which form ducts when the sheets are stacked upon each other, an exciting coil connected to an electric current source, and a measuring coil connected to a voltage sensitive device, said coils being mounted in said ducts and the winding planes of said coils being arranged substantially perpendicularly to each other and the mechanical forces being applied to the measuring body in a direction substantially parallel to the plane of said sheets and at an angle of substantially 45 to the winding planes of said coils.

3. A means according to claim 2, in which the measuring body is provided with two oppositely disposed projections of smaller end width than said body for the application of the mechanical force to said body, said projections having a steadily increasing section towards said measuring body.

4. A means according to claim 3, in which a plurality of measuring bodies form a coherent unit.

5. A means of the character described for measuring mechanical forces, comprising a measuring body of magnetostrictive material, said measuring body being provided with four substantially parallel ducts, two coils each threaded through a pair of said ducts and linking said body so that the two coils form a cross-like pattern, means for applying said forces to said measuring body so that at least a component of force is exerted in a direction which lies in a plane at an angle of substantially 45 to the winding planes of said coils, an alternating current source connected to one of said coils, and a voltage sensing means connected to the other coil.

6. A means according to claim 5, in which said measuring body is a laminated magnetic structure composed of sheets of magnetic material bonded together by a synthetic resin, the planes of said sheets being substantially perpendicular to said ducts and substantially parallel to said mechanical stresses produced by said forces.

7. A means according to claim 6, in which the measuring body is provided with two oppositely disposed projections of smaller end width than said body for the application of the mechanical forces of said body, said projections having a steadily increasing cross-section towards said body.

8. A means according to claim 7, in which a plurality of measuring bodies form a coherent unit.

9. A means according to claim 1, in which said measuring body is built up from sheets of magnetic material, the planes of said sheets being substantially perpendicular to the winding planes of said coils and substantially parallel to the direction of said force.

References Cited in the file of this patent UNITED STATES PATENTS 1,666,680 Buckley Apr. 17, 1928 2,370,845 Davis Mar. 6, 1945 2,553,833 Rifenbergh May 22, 1951 2,557,393 Rifenbergh June 19, 1951 FOREIGN PATENTS 442,441 Great Britain Feb. 3, 1936 658,569 Germany Apr. 11, 1938 

